Water treatment systems have been in existence for many years. These systems treat stormwater surface runoff or other polluted water. Stormwater runoff is of concern for two main reasons: i. volume and flow rate, and ii. pollution and contamination. The volume and flow rate of stormwater runoff is a concern because large volumes and high flow rates can cause erosion and flooding. Pollution and contamination of stormwater runoff is a concern because stormwater runoff flows into our rivers, streams, lakes, wetlands, and/or oceans. Pollution and contamination carried by stormwater runoff into such bodies of water can have significant adverse effects on the health of ecosystems.
The Clean Water Act of 1972 enacted laws to improve water infrastructure and quality. Sources of water pollution have been divided into two categories: point source and non-point source. Point sources include wastewater and industrial waste. Point sources are readily identifiable, and direct measures can be taken to mitigate them. Non-point sources are more difficult to identify. Stormwater runoff is the major contributor to non-point source pollution. Studies have revealed that contaminated stormwater runoff is the leading cause of pollution to our waterways. As we build houses, buildings, parking lots, roads, and other impervious areas, we increase the amount of water that runs into our stormwater conveyance systems and eventually flows into rivers, lakes, streams, wetlands, and/or oceans. As more land becomes impervious, less rain seeps into the ground, resulting in less groundwater recharge and higher velocity flows, which cause erosion and increased pollution levels of watery environments.
Numerous sources introduce pollutants into stormwater runoff Sediments from hillsides and other natural areas exposed during construction and other human activities are one source of such pollutants. When land is stripped of vegetation, stormwater runoff erodes the exposed land and carries it into storm drains. Trash and other debris dropped on the ground are also carried into storm drains by stormwater runoff. Another source of pollutants is leaves and grass clippings from landscaping activities that accumulate on hardscape areas and do not decompose back into the ground, but flow into storm drains and collect in huge amounts in lakes and streams. These organic substances leach out large amounts of nutrients as they decompose and cause large algae blooms, which deplete dissolved oxygen levels in marine environments and result in expansive marine dead zones. Unnatural stormwater polluting nutrients include nitrogen, phosphorus, and ammonia that come from residential and agricultural fertilizers.
Heavy metals that come from numerous sources are harmful to fish, wildlife, and humans. Many of our waterways are no longer safe for swimming or fishing due to heavy metals introduced by stormwater runoff. Heavy metals include zinc, copper, lead, mercury, cadmium and selenium. These metals come from vehicle tires and brake pads, paints, galvanized roofs and fences, industrial activities, mining, recycling centers, etc. Hydrocarbons are also of concern and include oils, gas, and grease. These pollutants come from leaky vehicles and other heavy equipment that use hydraulic fluid, brake fluid, diesel, gasoline, motor oil, and other hydrocarbon based fluids. Bacteria and pesticides are additional harmful pollutants carried into waterways by stormwater runoff.
Over the last 20 years, the Environmental Protection Agency (EPA) has been monitoring the pollutant concentrations in most streams, rivers, and lakes in the United States. Over 50% of waterways in the United States are impaired by one of more of the above-mentioned pollutants. As part of the EPA Phase 1 and Phase 2 National Pollutant Discharge Elimination Systems, permitting requirements intended to control industrial and nonindustrial development activities have been implemented. Phase 1 was initiated in 1997 and Phase 2 was initiated in 2003. While there are many requirements for these permits, the main requirements focus on pollution source control, pollution control during construction, and post construction pollution control. Post construction control mandates that any new land development or redevelopment activities incorporate methods and solutions that both control increased flows of surface water runoff from the site and decrease (filter out) the concentration of pollutants off the site. These requirements are commonly known as quantity and quality control. Another part of these requirements is for existing publicly owned developed areas to retrofit the existing drainage infrastructure with quality and quantity control methods and technologies that decrease the amount of surface water runoff and pollutant concentrations therein.
One approach to treat stormwater known in the art is separation or sedimentation of materials that are not neutrally buoyant, as an example, particles that are heavier and trash and oils that are lighter. Stormwater treatment devices that provide this method of treatment are called hydrodynamic separators. A hydrodynamic separator, or settling hydrodynamic separator, controls the flow of water by reducing incoming velocities and maximizing flow of the flow path which allows trash, debris, and total suspended solids (TSS), floatables and oils to settle out of the stormwater and remain contained within the hydrodynamic separator for later removal.